I've come to the conclusion that prone effects can become bugged for some characters.  Aloth in my savegame file always inflcits a prone of duration 0.0 seconds. Now that I realized that after seeing that he always inflicts the 0.0 duration effect from both items and spells, I tested other characters.  I see now that Eder also has this bug and will always, regardless of item or ability, inflict a 0.0 duration prone effect for any item or ability that causes prone.

 

I can't seem to remove this bug from them once they have it.  Very annoying.

Here's a savegame file for you to see.

 

https://www.dropbox.com/s/e602n9lm75bcco4/446fece33a0e48eb8895d9f4e9b575f3%2015103878%20Cave.savegame?dl=0